Highly porous metal-organic framework liquids and glasses via a solvent-assisted linker exchange strategy of ZIF-8

By combining the porosity of crystalline metal-organic frameworks (MOFs) with the unique processability of the liquid state, melt-quenched MOF glasses offer exciting opportunities for molecular separation. However, progress in this field is limited by two factors. Firstly, only very few MOFs melt at elevated temperatures and transform into stable glasses upon cooling the corresponding MOF liquid. Secondly, the MOF glasses obtained thus far feature only very small porosities and very small pore sizes. Here, we demonstrate solvent-assisted linker exchange (SALE) as a versatile method to prepare highly porous melt-quenched MOF glasses from the canonical ZIF-8. Two additional organic linkers are incorporated into the non-meltable ZIF-8, yielding high-entropy, linker-exchanged ZIF-8 derivatives undergoing crystal-to-liquid-to-glass phase transitions by thermal treatment. The ZIF-8 glasses demonstrate specific pore volumes of about 0.2 cm3g–1, adsorb large amounts of technologically relevant C3 and C4 hydrocarbons, and feature high kinetic sorption selectivities for the separation of propylene from propane.

This manuscript has been previously reviewed at another journal that is not operating a transparent peer review scheme.This document only contains reviewer comments and rebuttal letters for versions considered at Nature Communications.

REVIEWER COMMENTS
Reviewer #1 (Remarks to the Author): The authors have modified their manuscript to address many of my previous questions and concerns.This manuscript is likely suitable for publication in Nature Communications, but it would be ideal if the authors could more adequately address the following points: 1) It is still not particularly convincing that ZIF-8-mim0.15im0.74bim0.11undergoes a "melting" transition (as opposed to amorphization).This is important as the authors underscore that SALE technique enabled the melting transition of pristine sodalite ZIF structure for the first time.The extremely small ∆Hfus and ∆Sfus (0.7 kJ/mol and 1.03 J/K•mol, respectively) is particularly concerning.In Supplementary Figure 64 (DSC of ZIF-8-mim0.18im0.72bim0.10) and Supplementary Figure 80 (DSC of ZIF-8-mim0.20im0.70Clbim0.10),there is an exothermic feature before the endothermic feature assigned to melting.Based on the authors' explanation and the DSC results, the exothermic framework collapse and the endothermic melting transition seem to occur simultaneously for ZIF-8-mim0.15im0.74bim0.11.In this case, the measured ∆H of this transition should include the exothermic ∆H for the framework collapse, thus the actual ∆Hfus and ∆Sfus of this melting transition should be larger than the measured ∆H, which seems more reasonable.Additionally, can the authors comment on if the heat capacity changes for ag-ZIF-8-mim0.15im0.74bim0.11around the glass transition temperature are comparable to the reported values for a glass-to-liquid transition (e.g., 0.11 J/g•K for IL@ZIF-8, Nat.Commun. 2021, 12, 5703)?
2) With respect to propylene/propane selectivity, both diffusivity and solubility are important for membrane applications, and it is difficult to determine whether or not this glass might actually be useful for membrane applications.Moreover, mixed-gas diffusivity differences could be different than pure-component ones.
Reviewer #2 (Remarks to the Author): In reviewing this manuscript again I have calibrated my comments to the obvious fact that the bar for novelty and significance of a paper is much, much lower for Nature Communications than for Nature Materials (the original journal).
Re-reading my own comments and those of the other reviewers I think it was the correct decision to reject the paper from Nature Materials.However, for Nature Communications this is a much closer decision, and indeed I probably come down in favour of acceptance of this particular submission.
The main reason for initial rejection was that the conceptual novelty of the work was compromised by previous publications (some by the same group).All referees pointed this out.For Nature Communications I think the main question becomes whether the increase in porosity (almost twofold) is significant enough in itself to merit publication.I think that for Nature Communications this is a significant enough result.Increasing the porosity of such glassy materials is clearly of great importance for many (although not all) of their potential applications.The fact that a major jump in porosity is possible by changing composition will be of interest to a wide range of the readership interested in framework materials.
There was never really a question about the technical merit of the paper and the questions that were raised (e.g. the weak DSC signals) have been answered suitably by the authors.Therefore I think that the paper is suitable for publication in Nature Communications.
Reviewer #1 (Remarks to the Author): The authors have modified their manuscript to address many of my previous questions and concerns.This manuscript is likely suitable for publication in Nature Communications, but it would be ideal if the authors could more adequately address the following points:

Response:
We thank the reviewer for the positive assessment of our revised manuscript.
1) It is still not particularly convincing that ZIF-8-mim0.15im0.74bim0.11undergoes a "melting" transition (as opposed to amorphization).This is important as the authors underscore that SALE technique enabled the melting transition of pristine sodalite ZIF structure for the first time.The extremely small ∆Hfus and ∆Sfus (0.7 kJ/mol and 1.03 J/K•mol, respectively) is particularly concerning.In Supplementary Figure 64 (DSC of ZIF-8-mim0.18im0.72bim0.10) and Supplementary Figure 80 (DSC of ZIF-8-mim0.20im0.70Clbim0.10),there is an exothermic feature before the endothermic feature assigned to melting.Based on the authors' explanation and the DSC results, the exothermic framework collapse and the endothermic melting transition seem to occur simultaneously for ZIF-8-mim0.15im0.74bim0.11.In this case, the measured ∆H of this transition should include the exothermic ∆H for the framework collapse, thus the actual ∆Hfus and ∆Sfus of this melting transition should be larger than the measured ∆H, which seems more reasonable.

Response:
This is a very interesting point.Given that macroscopic flow is evident for agZIF-8-mim0.15im0.74bim0.11, the parent material ZIF-8-mim0.15im0.74bim0.11 is clearly in a liquid state when heated across the melting point.Indeed, framework collapse and melting happen simultaneously.Hence, it is not possible to separate the two events.It is reasonable that the framework collapses because of the metal-linker-bond dissociation associated with melting.So, framework collapse and melting are closely intertwined and likely not independent events (at least on the timescale of the current experiments).The same is observed for ZIF-62 and TIF-4, which both undergo densification (i.e. a partial framework collapse) upon melting (see Nat. Commun. 2022, 13, 7750).As outlined in our manuscript, the magnitude of densification at the solid-liquid transition is much more significant for ZIF-8-mim0.15im0.74bim0.11than for ZIF-62 and TIF-4.Thus, it is reasonable that the enthalpy and entropy of fusion of ZIF-8-mim0.15im0.74bim0.11are substantially smaller than the ones of ZIF-62 and TIF-4.

Response:
Thank you for this suggestion.We now determined the heat capacity (Cp) of agZIF-8-mim0.15im0.74bim0.11 in the temperature range from 200 to 415 °C and we compared the heat capacity change (∆Cp) of agZIF-8-mim0.15im0.74bim0.11around its glass transition (Supplementary Figure 82 in the revised Supplementary Information File) with other reported representative ZIF glasses (Supplementary Table 5 in the revised Supplementary Information File).∆Cp amounts to 0.12 J g -1 K -1 and is similar to that of ag(IL@ZIF-8-HT) (∆Cp = 0.11 J g -1 K -1 ) and that of other ZIF glasses previously reported.We added a new section with the heat capacity data to the Supplementary Information File (Supplementary Methods 6.2, page 52 in the revised Supplementary Information File) and an additional statement to the revised manuscript.
Addition to the manuscript text (page 10):